Possibly around 500 lb. But I also have had no need for 120 or more fuel tanks for years. Max I spec is 100's. Probably could get by fine with 80's. Probably will go with 80's next time. Most of the time I fuel, it is usually between 100 and 120 gallons at a time. Quite a bit of weight can be saved by not having large fuel tanks. More critical back when 5 mpg was norm and folks idled all night every night.
I have a 2015 company spec Cascadia Evolution...this thing is a monster as far as holding heavy loads back...Has anyone had to fight with the dt 12 to get it to creep mode while backing up? I can on level ground, but if theres a slight bump it detects, itll cancel creep mode. Its pretty annoying if anything